in the Carboniferous rocks, whole strata being often made up of little else than the little bivalved shells of these Crustaceans.  Many of them are extremely small, averaging about the size of a millet-seed; but a few forms, such as Entomoconchus Scouleni (fig. 122, c), may attain a length of from one to three quarters of an inch.  The old group of the Phyllopods is is likewise still represented in some abundance, partly by tailed forms of a shrimp-like appearance, such as Dithyrocaris (fig. 122, d), and partly by the curious striated Estherioe and their allies, which present a curious resemblance to the true Bivalve Molluscs (fig. 122, b).  Lastly, we meet for the first time in the Carboniferous rocks with the remains of the highest of all the groups of Crustaceans—­namely, the so-called “Decapods,” in which there are five pairs of walking-limbs, and the hinder end of the body ("abdomen”) is composed of separate rings, whilst the anterior end is covered by a head-shield or “carapace.”  All the Carboniferous Decapods hitherto discovered resemble the existing Lobsters, Prawns, and Shrimps (the Macrura), in having a long and well-developed abdomen terminated by an expanded tail-fin.  The Paloeocaris typus (fig. 122, e) and the Anthrapaloemon gracilis (fig. 122, f), from the Coal-measures of Illinois, are two of the best understood and most perfectly preserved of the few known representatives of the “Long-tailed” Decapods in the Carboniferous series.  The group of the Crabs or “Short-tailed” Decapods (Brachyura), in which the abdomen is short, not terminated by a tail-fin, and tucked away out of sight beneath the body, is at present not known to be represented at all in the Carboniferous deposits.

[Illustration:  Fig. 123.—­Cyclophthalmus senior.  A fossil Scorpion from the Coal-measures of Bohemia.]

[Illustration:  Fig. 124.—­Xylobius Sigillarioe, a Carboniferous Myriapod. a, A specimen, of the natural size; b, Anterior portion of the same, enlarged; c, Posterior portion, enlarged.  From the Coal-measures of Nova Scotia. (After Dawson.)]

[Illustration:  Fig. 125—­Haplophlebium Barnesi, a Carboniferous insect, from the Coal-meastures of Nova Scotia. (After Dawson.)]

In addition to the water-inhabiting group of the Crustaceans, we find the articulate animals to be represented by members belonging to the air-breathing classes of the Arachnida, Myriapoda, and Insecta.  The remains of these, as might have been expected, are not known to occur in the marine limestones of the Carboniferous series, but are exclusively found in beds associated with the Coal, which have been deposited in lagoons, estuaries, or marshes, in the immediate vicinity of the land, and which actually represent an old land-surface.  The Arachnids are at present the oldest known of their class, and are represented both by true Spiders and Scorpions.